Filter

ABSTRACT

A full duplex radio having improved properties is obtained by using asymmetric surface acoustic wave (SAW) filters, the filters are composed of series and parallel coupled SAW resonators. Asymmetry is obtained by covering either of the series or parallel resonators of each filter with a dielectric layer to increase the SAW coupling coefficient of the covered resonators relative to the uncovered resonators. The filters are desirably in pairs arranged with mirror image frequency asymmetry such that the steeper skirts of the frequency response are adjacent. Greater pass-bandwidths can be obtained without adverse affect on transmitter and receiver isolation.

FIELD OF THE INVENTION

The present invention relates to means and methods for improved radiosand filters, and more particularly, improved radios and filtersemploying Surface Acoustic Wave (SAW) devices.

BACKGROUND OF THE INVENTION

Surface acoustic wave (SAW) devices are much used today in electroniccommunication, especially SAW devices arranged to provide filteringfunctions. Filters formed from SAW devices are particularly useful inconnection with portable radio and telephones. Such radios telephonesoperate typically in the 500-1500 MHz range and higher.

Filter characteristics that are of particular interest to the radiodesigner are: (a) the pass-bandwidth, that is, the range of frequencieswithin which the filter passes a signal with acceptable loss, (b) thepass-band attenuation, that is, how much loss occurs in the pass-band,(c) the transition bandwidth, that is, the range of frequencies whichseparate the pass-band and the stop-band, and (d) the stop-bandattenuation, that is, the attenuation outside the pass-band andtransition-band where no signal is desired to be transmitted. The stopband is the frequency region in which the filter provides very highattenuation. The transition band is the frequency region in which theattenuation rapidly increases from a low value (little attenuation) atthe corner frequency of the pass-band to a high value (largeattenuation) in the stop-band. The transition band is also referred toin the art as the "skirt" of the filter transfer characteristic, e.g.,the fall-off region on either side of the pass-band on a plot of filterattenuation versus frequency.

It is a feature of SAW filters that the widths of the pass-band andtransition-band are related. The choice and cut of piezoelectricsubstrate material from which the SAW filter is constructed and theelectrode shape, spacing and location influence the characteristics ofthe SAW filter. Design modifications which cause the pass-band toincrease in width generally also cause the transition-bandwidth toincrease. Conversely, those choices which allow one to obtain smalltransition-bandwidths also yield narrow pass-bandwidths. This makes itextremely difficult to design SAW filters which simultaneously provide arelatively wide pass-bandwidth but at the same time have very narrowtransition-bandwidths, that is, steep skirts on the filter transfercharacteristic.

It is an advantage of the present invention that it overcomes theselimitations of the prior art so that a greater pass-bandwidth isobtained while preserving a narrow transition-bandwidth (steep skirt) onat least one side of the pass-band. Such a SAW filter is advantageouslyused to improve the properties of radios.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a radio utilizing one or moreimproved SAW filters of the present invention;

FIG. 2 is a simplified schematic top view of an elemental portion of aSAW filter, illustrating a SAW electrode structure in simplified form;

FIG. 3 is a simplified electrical equivalent circuit of the physicalstructure of FIG. 2;

FIG. 4 illustrates in simplified form, how multiple elemental structuresof FIGS. 2-3 are combined in a series-parallel arrangement to produce aSAW filter;

FIG. 5 shows an illustrative SAW filter transfer characteristic for thestructure of FIG. 4 wherein the SAW coupling coefficients of the seriesand parallel resonators are substantially the same;

FIG. 6 is a simplified plan view of a SAW filter according to thepresent invention wherein elemental devices of FIGS. 2-3 are arranged inseries-parallel combination and the parallel resonators are modified toincrease their SAW coupling coefficient;

FIG. 7 shows an illustrative SAW filter transfer characteristic for thestructure of FIG. 6;

FIG. 8 is a simplified plan view of a SAW filter similar to that of FIG.6 but wherein the series resonators are modified to increase their SAWcoupling coefficient;

FIG. 9 shows an illustrative SAW filter transfer characteristic for thestructure of FIG. 8;

FIG. 10 shows a structure similar to that illustrated in FIG. 8 butwhere only a portion of the series resonators have been modified;

FIG. 11 shows a simplified cross-section through the elemental device ofFIG. 2 illustrating how the SAW coupling coefficient of a SAW resonatoris modified by use of an overlayer above the electrode fingers;

FIG. 12 shows a simplified plan view of the electrode fingers ofmultiple elemental resonators as in FIG. 2, coupled to form a devicesimilar to that shown in FIGS. 4, 6, 8, 10, but with a couplingcoefficient modification analogous to that shown in to FIG. 8;

FIG. 13 is a simplified flow chart illustrating the process ofmanufacture of the improved SAW device of the present invention; and

FIG. 14 is a plot of attenuation versus frequency for two SAW filters,one having a response of the type illustrated in FIG. 7 and the otherhaving a response of the type illustrated in FIG. 9, showing how theyare arranged in frequency to simultaneously provide broader bandwidthsand high isolation between closely spaced interfering signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematic block diagram of radio 10 embodying one or moreimproved SAW filters according to the present invention. Radio 10comprises antenna 12 which is coupled by line 14 to Transmit/Receiver(T/R) filter 16. The purpose of T/R filter 16 is to help provideisolation between receive frequencies (F_(rec)) and transmit frequencies(F_(trans)) so that duplex communication can occur, i.e., simultaneousreception and transmission.

T/R filter 16 conveniently has two sub-portions, T/R receive filter 16'and T/R transmit filter 16". Filter 16' is tuned to pass the receivefrequencies F_(rec) to line 17 and receive channel SAW filter 18 whileattenuating transmit frequencies F_(trans) coming from line 48 andtransmit power amplifier 46. Filter 16" is tuned to pass the transmitfrequencies (F_(trans)) on line 48 from transmit power amplifier 46 andattenuate any sidebands generated within amplifier 46 that might overlapF_(rec). T/R filter 16 can comprise one or both of sub-filters 16', 16".For example, when T/R filter 16' is omitted, receive SAW filter 18 iscoupled directly to antenna lead 14 as shown by alternative lead 17'. Inthis circumstance, T/R filter 16 contains only filter 16" tuned toF_(trans).

The incoming signal is passed via lines 17 or 17' to receive SAW filter18 which is coupled by line 20 to pre-amplifier 22 which is in turncoupled by line 24 to receiver 26. Receiver 26 demodulates the amplifiedincoming signal and delivers it via line 28 to input/output (I/O) device30 which includes for example, an annunciator or other digital or analogoutput device. In a communications transceiver intended for voicecommunications, I/O device 30 will generally include a microphone,speaker or headphone, display and other conventional circuitry forvolume control and signal processing. When digitized voice transmissionis employed, I/O device 30 will also generally include a CODEC. Personsof skill in the art will understand that many different functions can beincluded in I/O device 30 depending upon the nature of the informationdesired to be received and transmitted. Non-limiting examples ofdifferent types of information are voice, video, fax and data.

I/O device 30 is coupled to transmitter section 32 by line 34 so thatsignals originating within I/O device 30 can be broadcast by radio 10.The output of transmitter section 32, usually comprising a modulatedradio frequency (RF) signal, is coupled via line 36 to pre-amplifier 38and via line 40 to transmit SAW filter 42, and thence by line 44 tofinal power amplifier 46, by line 48 to T/R filter 16, and then by lead14 to antenna 12 from which the signal provided by I/O device 30 istransmitted in radio frequency (RF) modulated form to another radio. Anyform of modulation or coding well known in the art may be employed byradio 10. Radio 10 is distinguished from the prior art by, among otherthings, the properties of filters 16, 18 and/or 42 which are describedin more detail below. Filters 16, 18, 42 can be formed as separate SAWfilters or can be on a common substrate as indicated by dashed line 15or a combination thereof.

FIG. 2 is a simplified schematic top view of elemental SAW resonator ordevice 50 which is used to form a SAW filter. Elemental SAW resonator 50comprises piezoelectric substrate 52 on which are formed interleavedelectrodes 54, 56 having, respectively, connection points 55, 57.Substrate 52 may be made of any piezoelectric material, but quartz,lithium niobate and lithium tantalate are particularly useful. Crystalsof these materials cut at different angles are employed, according totechniques well known in the art. Electrodes 54, 56 are convenientlyformed of aluminum but other conductive materials can also be employed.Fabrication techniques for forming SAW resonators of predeterminedfrequency response are well known in the art.

FIG. 3 shows simplified electrical equivalent circuit 60 of elementalSAW resonator or device 50 of FIG. 2. Circuit 60 has connections 55, 57corresponding to the like identified connection points of device 50 ofFIG. 2. Circuit 60 comprises series resistance 62, inductance 64 andcapacitance 66, parallel capacitance 68 and parallel resistance 69.Resistances 62, 69 account for the signal loss encountered in thepass-band of the SAW device. Persons of skill in the art will understandthat elemental SAW device 50 represented by equivalent circuit 60 has aseries resonant frequency (f_(r)) at which the impedance betweenterminals 55, 57 becomes very small and a parallel resonant frequency(f_(a)), also referred to as the "anti-resonant" frequency, where theimpedance between terminals 55, 57 is very large. In general, f_(a)>f_(r).

FIG. 4 illustrates in simplified form, SAW filter 70 with input 72 andoutput 74, formed from multiple repetitions of similar SAW resonators ordevices 50, arranged in series-parallel connection. Series connectedresonators are denoted by the reference numbers 50s and parallel (i.e.,shunt) connected resonators are denoted by the reference numbers 50p.FIG. 5 shows attenuation versus frequency transfer characteristic 76 ofSAW filter 70. Each of series elements 50s and parallel elements 50p areformed from resonator 50 of FIG. 2. Individual series resonators 50s andparallel resonators 50p can be tuned to slightly different frequenciesto vary the shape of transfer characteristic 76. Persons of skill in theart will understand based on techniques well known in the art how tochoose the number of series-parallel stages and their individual valuesof f_(a) and f_(r) needed to achieve a desired pass-bandwidth andstop-bandwidth. It is often desirable to set f_(a) of shunt resonators50p about equal to f_(r) of series resonators 50s, as indicated at thetop of FIG. 5. These resonances substantially determined the frequencyof the pass-band of the filter.

Transfer characteristic 76 has pass-band 91 of pass-bandwidth width 77wherein attenuation is less than a predetermined amount indicated byhorizontal dashed line 75. Transfer characteristic 76 has lowertransition band 93 of bandwidth 78 corresponding to lower transition orskirt 79 between lower pass-band corner frequency 80 and lower stop-bandedge frequency 82 where the attenuation exceeds predetermined level 85.Frequency 82 corresponds approximately to f_(r) of parallel resonators50p. Stop-band 84 lies below frequency 82.

Transfer characteristic 76 has upper transition band 95 of bandwidth 86corresponding to upper transition or skirt 87 between upper pass-bandcorner frequency 88 and upper stop-band edge frequency 89 where theattenuation exceeds predetermined level 85. Frequency 89 correspondsapproximately to f_(a) of series resonators 50s. Stop-band 90 lies abovefrequency 89.

In the case of SAW filters composed of multiple SAW resonators 50 of thetype shown in FIG. 2, pass-band width 77 and transition bandwidths 78,86 are related and dependent on the SAW coupling coefficient K², whereK² is a property of the substrate material. Choosing a value of K² whichincreases pass-bandwidth 77, also increases transition bandwidths 78,86. Choosing a lower value of K² (e.g., by changing the material or cut)will make the transition bandwidths 78, 86 smaller, but also narrowpass-bandwidth 77. Choosing larger values of K² has the opposite effect,that is, widening both pass-bandwidth 77 and the transition-bandwidths78, 86. Thus, the designer of conventional SAW filters is oftenfrustrated by conflicting requirements for wide pass-bandwidths andnarrow transition-bandwidths. This is especially important in connectionwith radios where SAW filters are desired to be used to prevent spurioussignals from the transmitter section from interfering with the receiversection of a full-duplex radio, such as is illustrated in FIG. 1.

The above-noted difficulties are overcome by making the effectivecoupling coefficients K² different for different portions of the filterof FIG. 4. How this is done is illustrate in FIGS. 6-14. FIG. 6 is asimplified plan view of SAW filter 92 according to a first embodiment ofthe present invention wherein elemental devices or resonators of FIGS.2-3 are arranged in series-parallel combination (similar to thearrangement described in connection with FIG. 4) but with the parallelresonators 50p modified to increase their effective SAW couplingcoefficient K². Resonators 50s or 50p which have been modified toincrease their coupling coefficients are identified by addition of aprime symbol, as in 50s' or 50p'.

Filter 92, with input electrodes 721, 722 and output electrodes 741,742, is formed on piezoelectric substrate 94 such as has already beendescribed. Filter 92 comprises multiple series coupled resonators 50sand multiple parallel coupled resonators 50p', each analogous toelemental device 50, except that parallel resonators 50p' have beenmodified to have larger values of K² than the associated seriesresonators 50s. This is conveniently accomplished by overlying parallelresonators 50p with dielectric layer 96 (see also FIG. 11). Dielectriclayer 96 is conveniently an insulating material such as SiO₂ or TiO₂ orAl₂ O₃ or MgO or SiO or Si₃ N₄ or Ta₂ O₅ or ZnO or combinations thereofor other doped or undoped III-V or II-VI compounds well known in theart. Other insulating or semi-insulating materials may also be usedprovided that their presence does not short electrodes 54, 56 (see FIG.2). Layer 96 need not be of a piezoelectric material. As used herein,the term "glass" is intended to include any and all of the above-listedand equivalent materials.

Placing glass layer 96 on top of parallel resonators 50p has the effectof increasing the effective value of K² for those resonators whileleaving the series resonators unaffected. The resonant frequencies ofthe transducers covered by glass layer 96 will, in general, be shiftedslightly from their un-covered values. However, persons of skill in theart can readily determined the magnitude of the expected shift dependingupon the amount and physical properties of the glass being used so thatvalues of f_(a) and f_(r) for the resonators prior to covering them withglass can be offset. In this way f'_(a) and f'_(r) obtained aftercovering the resonators with layer 96 will have the desired values.

FIG. 7 shows an illustrative SAW filter transfer characteristic 76' forthe structure of FIG. 6. Upper stop-band 90, upper attenuation frequency89, upper transition width 86 and upper band-pass corner frequency 88are, relatively, undisturbed. In particular, transition bandwidth 86 issubstantially the same as for filter 70. However, the presence of layer96 on parallel resonators 50p' provides pass-band 91' of largerpass-bandwidth 77' and transition-band 93' of larger transitionbandwidth 78'. Lower pass-band corner frequency 80', and lower stop-bandcorner frequency 82' and stop-band 84' are moved to lower frequencies.Filter 92 provides a significant improvement over prior art filterswhere it is being used to filter out a powerful signal source which ishigher in frequency than the filter pass-band, without sacrifice ofpass-bandwidth. Thus, filter 92 is useful in radio 10 as filter 16'and/or 18 where f_(trans) is above f_(rec) or as filter 16" and/or 42where the f_(rec) is above f_(trans) (see FIG. 14).

FIG. 8 is a simplified plan view of SAW filter 100 similar to SAW filter92 of FIG. 6 but wherein series resonators 50s are modified to increasetheir SAW coupling coefficients, i.e., as 50s'. SAW filter 100 withinput connection 721, 722 and output connections 741, 742 has parallelelements 50p and series elements 50s' wherein series elements 50s' aresimilar to elements 50s in FIG. 6 but modified by the addition of layer96 (see FIG. 11) in substantially the same way as in connection withelements 50p' of FIG. 6. The effect of providing layer 96 is to increasethe coupling coefficients K². This has an effect shown in FIG. 9 whichillustrates transfer characteristic 76" corresponding to filter 100.

Transfer characteristic 76" has lower stop-band 84, lower stop-bandcorner frequency 82, lower transition skirt 79 and lower transition-band93 of bandwidth 78, analogous to that in FIG. 5, with transitionbandwidth 78 substantially the same. Increasing the effective couplingcoefficient K² of series elements 50s' has the effect of raising upperpass-band corner frequency 88", upper stop-band corner frequency 89" andstop band 90". Thus, passband 91" has increased bandwidth 77" andtransition band 95" has greater bandwidth 86". Filter 100 provides asignificant improvement over prior art filters where it is used tofilter out a powerful signal source which is lower in frequency than thefilter pass-band, without sacrifice of pass-bandwidth. Thus, filter 100is useful in radio 10 as filter 16' and/or 18 where f_(trans) is belowf_(rec) or as filter 16" and/or 42 where the f_(rec) is below f_(trans)(see FIG. 14).

FIG. 10 is analogous to FIG. 8 except that filter 100' has only some ofseries elements 50s' covered by layer 96 so as to provide propertiesintermediate between those of filter 70 (FIGS. 6, 7) and filter 100(FIGS. 8, 9). The number of series resonators to be provided with theincrease in K² will depend upon the particular characteristic desired bythe user.

While the modifications to filters 92, 100 relative to filter 70 havebeen described in terms of raising K² of certain series or parallelresonators, those of skill in the art will understand based on thedescription herein that while the described arrangement is particularlyconvenient, what is important is providing a difference of K² valuesbetween the series resonators and the parallel resonators of the samefilter. This can be accomplished by either raising or lowering K² valuesof series resonators relative to parallel resonators or vice-versa sothat a difference exists.

FIG. 11 shows a simplified cross-sectional view of resonator 50 of FIG.2 illustrating how layer 96 interacts with electrode fingers 54, 56. Ina typical device intended for operation in the range of about 700-1000MHz and more typically about 945 MHz (e.g., an acoustic wavelength about4 microns), electrode fingers 54, 56 have width 102 of about 25% of theacoustic wavelength, spacings of about 25% of the acoustic wavelength,and thickness 106 of about 0.1-0.15 microns, but larger or smallervalues can also be used. Substrate 52 typically has a thickness 108 inthe range of about 0.5 mm, more or less. Thickness 110 of layer 96 isconveniently about 10%-75% of the acoustic wavelength with about 25%being preferred. Layer 96 need not be substantially uniform in thicknessalthough this is preferred.

FIG. 12 shows filter 112 illustrating in greater detail how electrodes54, 56 of elemental structures 50s, 50p are conveniently arranged onsubstrate 114 and coupled to form a ladder structure analogous to thatshown in FIGS. 4, 6, 8, 10, but with the coupling coefficientmodification analogous to that of FIG. 8, i.e., series resonatingelements covered by layer 96. It will be apparent from FIG. 12 that avery compact structure can be obtained by appropriate arrangement ofelectrodes 54, 56 into series devices 50s' and parallel devices 50p sothat common lines are shared. This arrangement can also be used toconstruct the other filters described herein with different ones of theseries (50s) or parallel (50p) resonators modified to have differentvalues of K².

FIG. 13 shows flow chart 120 illustrating the practice of the method ofthe present invention for forming the filters described herein. In step122, a substrate wafer containing multiple SAW devices is prepared in aconventional way using means well known in the art. The SAW electrodesare deposited and patterned. In step 124 a glass, such as has beenpreviously described, is deposited over the electrode structure. Thisdeposition may be localized using shadow masks so as to fall only on thedesired areas or as is assumed in flow chart 120, depositedsubstantially uniformly over the whole area. Optional step 126 isprovided where the nature of the chosen glass makes it desirable toprovide a "priming" coating to increase the adhesion of the maskingmaterial (e.g., a photoresist) to the glass. In step 128 the etch resistis applied and patterned using conventional techniques and developed toexpose those areas where, usually, glass material is desired to beremoved. In step 130, the exposed glass is removed and the remainingresist stripped. While the above-described procedure is particularlyconvenient, those of skill in the art will understand based on thedescription herein that there are other techniques by which selectedtransducers in the series-parallel array may be covered with a layer toincrease the coupling coefficient of such transducer and thereby modifytheir resonant frequencies to obtain the transfer characteristicsdescribed above. A non-limiting example is lift-off techniques wellknown in the art.

FIG. 14 is a plot of transfer characteristic 140 (i.e., attenuation vs.frequency) for two SAW filters 92, 100 according to the presentinvention, one having response 76' of the type illustrated in FIG. 7 andthe other having response 76" of the type illustrated in FIG. 9. FIG. 14shows how combined pass-bands 91', 91" are arranged in relativefrequency for operation in radio 10. Pass-band 91' is arranged to belower in frequency than pass-band 91", with, for example, pass-band 91'centered on receiver frequency F_(rec) and pass-band 91" centered ontransmitter frequency F_(trans) of duplex radio 10 (note that therelative frequency of the transmitter and receiver can be interchanged).This places comparatively steep skirt portions 87, 79 closest togetherin frequency and more gradual skirt portions 79", 87" furthest apart infrequency.

Assuming that F_(rec) <F_(trans), then filters 16' and 18 of radio 10 ofFIG. 1 are desirably of type 92 illustrated in FIGS. 6-7 and filters16", 42 are desirably of type 100 illustrated in FIGS. 8-9. Conversely,assuming that F_(trans) F_(rec), then filters 16', 18 of radio 10 ofFIG. 1 are desirably of type 100 illustrated in FIGS. 8-9 and filters16", 42 are desirably of type 92 illustrated in FIGS. 6-7. Transfercharacteristic 140 illustrated in FIG. 14 is highly desirable forconstruction of duplex radios of improved performance.

Another manner of comprehending the present invention is to consider therelative coupling coefficient C=K² of the resonators where C_(s)represents the K² values of the series resonators 50s and C_(p)represents the K² values of the parallel resonators 50p. Thus, inapproximate terms: (i) for filter 70 with transfer characteristic 76,C_(s) =C_(p) so that C_(s) /C_(p) =1; for filter 92 with transfercharacteristic 76', C_(s) <C_(p) so that C_(s) /C_(p) <1; and for filter100 with transfer characteristic 76", C_(s) >C_(p) so that C_(s)/C_(p) >1. Accordingly, then transfer characteristics of the filtersused in radio 10 and shown in FIG. 14 can be expressed as using for thelower of F_(rec) or F_(trans) a filter with C_(s) /C_(p) <1 and usingfor the higher of F_(rec) or F_(trans) a filter with C_(s) /C_(p) >1. Itcan be seen from FIGS. 1 and 14 that when F_(rec) /F_(trans) <1, receiveSAW filters 16', 18 should have C_(s) /C_(p) <1 and transmit SAW filters16", 42 should have C_(s) /C_(p) >1. Conversely, when F_(trans) /F_(rec)<1, then transmit SAW filters 16", 42 should have C_(s) /C_(p) <1 andreceive SAW filters 16', 18 should have C_(s) /C_(p) >1.

By now it will be appreciated that there has been provided an improvedmeans and method for radios embodying SAW filters of improved propertiesand improved SAW filters themselves. The improved devices provide in thesame filter comparatively wider pass-bandwidths while at the same timepreserving the steepness of the low-high attenuation transition zone inthe transfer characteristic on one side of the pass-band while allowingthe transition zone on the opposite side of the pass-band to become lesssteep, where such decrease in steepness does not adversely affect radioperformance. It will be further apparent based on the teachings hereinthat by using the above-described filters in pairs wherein the lowerpass-band frequency filter has its steeper transition skirt on thehigher frequency side of the pass-band, and the higher pass-bandfrequency filter has its steeper transition skirt on the lower frequencyside, of its pass-band, that a radio of improved performance can beobtained. These filter pairs can be constructed on the same or differentsubstrates.

We claim:
 1. A method for making a SAW filter having asymmetricfrequency characteristics, said method comprising steps of:forming on asubstrate a combination of series coupled and parallel coupled SAWresonators; and disposing a dielectric layer on a portion of either butnot both of said series and parallel coupled SAW resonators.
 2. A methodas claimed in claim 1, wherein said step of forming includes a step offorming, on a substrate chosen from a group consisting of lithiumniobate, lithium tantalate or quartz, a combination of series coupledand parallel coupled SAW resonators.
 3. A method as claimed in claim 2,wherein said step of disposing includes a step of disposing a dielectriclayer chosen from a group consisting of silicon dioxide or monoxide,titanium dioxide, aluminum oxide (Al₂ O₃), magnesium oxide, siliconnitride, tantalum oxide, zinc oxide and II-VI compounds on said portionof either but not both of said series and parallel coupled SAWresonators.
 4. A method as claimed in claim 1, wherein said step ofdisposing includes a step of disposing a dielectric layer chosen from agroup consisting of silicon dioxide or monoxide, titanium dioxide,aluminum oxide (Al₂ O₃), magnesium oxide, silicon nitride, tantalumoxide, zinc oxide and II-VI compounds on said portion of either but notboth of said series and parallel coupled SAW resonators.
 5. A SAW filtermanufactured by the method of claim
 3. 6. A method for making a SAWfilter, and a SAW filter made by said method, said method comprisingsteps of:forming, on a substrate chosen from a group consisting oflithium niobate, lithium tantalate or quartz, a combination of seriescoupled and parallel coupled SAW resonators; and disposing a dielectriclayer chosen from a group consisting of silicon dioxide or monoxide,titanium dioxide, aluminum oxide (Al₂ O₃), magnesium oxide, siliconnitride, tantalum oxide, zinc oxide and II-VI compounds on a portion ofeither but not both of said series and parallel coupled SAW resonators.